GENERATING DEVONIAN-MISSISSIPPIAN BLACK-SHALE REPOSITORIES IN THE APPALACHIAN BASIN: REFINING THE TECTONIC MODELS

Since the 1980s, it has been clear that the origin of Devonian-Mississippian black-shale repositories in the Appalachian foreland basin was largely a flexural response to Acadian/Neoacadian orogeny, involving oblique convergence of various terranes with the southeastern margin of Laurussia. These terranes were loosely identified as “Avalonia,” but recent work indicates that the Carolina and other terranes were also involved. In fact, Carolina began its convergence with Laurussia in latest Silurian-earliest Devonian time with northeasterly sinistral transpression. By latest early Devonian time, northeastward dispersal of Carolina apparently halted due to encroachment by Avalonia, marking initiation of the Acadian/Neoacadian Orogeny. Initially, Carolina was caught in a pincer movement between Gondwana and Laurussia, generating a head-on collision between Carolina and the New York and Virginia promontories, which in two tectophases produced the Esopus/Needmore and Marcellus black-shale basins, respectively. In the latest Middle Devonian to Late Devonian third tectophase, oblique subduction of Laurussia below Carolina closed the gap between Carolina and Laurussia in a dextral, scissors-like fashion, resulting in five black-shale foreland basins that progressively migrated southwestward, tracking the closure. The fourth tectophase, now called the Neoacadian Orogeny, apparently represents the collision of Avalonia with the New York promontory and Carolina under the impact of Gondwana. As Avalonia was ejected in a pincer movement between Laurussia and Gondwana and collided with Carolina, its dextral shear was transferred to Carolina, producing a transpressional regime with the Virginia promontory that generated the Sunbury black-shale basin and the overlying Borden-Grainger-Price-Pocono and Pennington-Mauch Chunk clastic wedges. The Sunbury black-shale basin and overlying clastic wedges are among the largest and most extensive in the Appalachian Basin, reflecting the intensity and long duration (41 Ma) of the Neoacadian event. Although the story has grown more complex, the close association of black-shale basins with episodic tectonism suggests that in many places the tectonic contribution of generating basin repositories was critical in the formation of major black-shale deposits.